This invention relates to ground proximity warning systems, and more particularly to such systems which provide a warning in an aircraft when the aircraft is descending at an excessive rate.
A ground proximity warning system is defined by Federal Aviation Administration (FAA), specification RTCA DO-161A to provide a warning of dangerous imminent contact of an aircraft with ground. The specification defines an operational Mode 1 to provide a warning whenever the aircraft is descending at an excessive rate during approach maneuvers. Briefly, a Mode 1 warning is sounded when the combination of barometric altitude descent rate and height above the ground exceeds a predetermined envelope. The envelope is shown on a graph having barometric altitude sink rate in feet per minute as the abcissa and the height above terrain (ground) in feet as ordinate. The envelope within which the warning is to be sounded is circumscribed by a horizontal line at 50 feet height and three straight line segments. The first segment extends from a point at 50 feet height and about 1,300 feet per minute sink rate to a point at about 1,250 feet height and about 2,200 feet per minute sink rate, a second line segment from the last mentioned point to a point at about 2,450 feet height and about 4,900 feet per minute sink rate and a third segment which is horizontal at about 2,450 feet height.
Wind shear is the manifestation of the meteorological situation wherein an upwelling column of air deflects horizontally outward in all directions from a common locus. An object such as an aircraft passing through a wind shear will experience about a 180 degree wind shift. In particular, an aircraft passing through a wind shear will experience a rapid change in air speed caused by a shift from head-wind to tail-wind conditions. It can be understood that when the aircraft passes through the wind shear the rapid change from head-wind to tail-wind conditions will result in a sudden and substantial loss of lift for the aircraft which may cause excessive aircraft descent rate so that eventually a Mode 1 ground proximity warning system warning will be sounded. In response to such a warning a pilot will increase power to his engines to regain the lost lift. It would be desirable to sound the Mode 1 warning somewhat sooner than would be otherwise normal when the aircraft passes through a wind shear and that the more severe the wind shear, the sooner the warning should be sounded.